Year	Years
1970	3.34
1971	3.44
1972	3.54
1973	3.63
1974	3.74
1975	3.84
1976	3.95
1977	4.05
1978	4.16
1979	4.28
1980	4.39
1981	4.51
1982	4.63
1983	4.75
1984	4.87
1985	5
1986	5.12
1987	5.25
1988	5.39
1989	5.52
1990	5.66
1991	5.79
1992	5.94
1993	6.08
1994	6.22
1995	6.37
1996	6.52
1997	6.67
1998	6.82
1999	6.97
2000	7.12
2001	7.28
2002	7.44
2003	7.6
2004	7.76
2005	7.92
2006	8.08
2007	8.24
2008	8.41
2009	8.57
2010	8.74
2011	8.9
2012	9.07
2013	9.24
2014	9.41
2015	9.58